1. Field of the Invention
The present invention relates to a system for determining the position of an elongate element of great length towed when submerged by a ship which uses a plurality of means for measuring the position in the water of the elongate element, distributed therealong.
More particularly, the invention relates to a system for determining the position of a marine seismic streamer comprising a plurality of means for measuring the direction and depth spaced apart along the whole of the seismic streamer and connection means disposed inside the elongate element for transferring the measurements obtained to a central recording and processing system installed in the ship.
2. Description of the Prior Art
One known method for determining the real position in the water of a seismic streamer formed from a plurality of interconnected sections each containing an array of seismic sensors and transmission lines, consists in incorporating inside the streamer at regular intervals direction measuring elements such as compasses, and elements, for measuring the hydrostatic pressure, such as manometers or pressure gauges, so as to obtain the orientation of the seismic streamer and the depth at which it is immersed at a plurality of locations and, consequently, so as to determine its position behind the towing ship. The measurements made by these elements are transmitted to a central recording and processing system disposed in the ship and recorded at the same time as the seismic signal picked up so that, during processing of the seismic recordings, the possible lateral drift of the seismic streamer with respect to the direction followed by the towing ship may be taken into account or the seismic profile plane corresponding to the recordings may be defined. Such a system for determining the position in the water of a seismic streamer is described for example in French patent No. 2.218.571.
Since the readings given by compasses are falsified by the presence of these compasses in the vicinity of magnetic masses, they are preferably disposed in positions remote from the metal parts of the connections joining together the ends of the different sections of the seismic streamer. In a first arrangement, the compasses are disposed in the central part of each streamer section and are connected to a conducting line. The case of each compass is held in position for example by the outer sheath of the streamer into which it is inserted at the time of its installation, but when this outer sheath is inflated so as to counterbalance the hydrostatic pressure at the depth where it is to operate and when the diameter of the sheath increases, for this reason, the initial position of the compass may be modified in an unpredictable way, which creates an uncertainty as to the true direction of the streamer section measured by the compass.
The arrangement of the measuring elements inside the streamer sections presents other disadvantages. Accessibility thereto is difficult and if, during use, operation thereof proves defective, it is necessary to completely change the corresponding streamer section.
Furthermore, in the case where the measuring elements comprise pressure gauges, their arrangement inside the sheath makes the preliminary operations for testing correct operation more difficult. It must also be emphasised that, since the sections of the seismic streamer must be interchangeable, it is necessary to incorporate measuring elements inside each of them, which appreciably increases their cost, particularly when these elements comprise a compass.
According to a second arrangement, the compasses and other measuring elements are incorporated in streamer sections or rigid cases inserted between the streamer sections. Each case is made from a non-magnetic material and its length is chosen to be fairly large so as to avoid the disturbances caused to the compasses by the metal parts of the connections between the streamer sections. Positioning of the compasses is in this case more accurate and the direction of the axis of the case is in this case known with greater accuracy. In addition, maintenance may be more readily carried out by replacing a section which proves to be defective. However, arrangement also has drawbacks. The other measuring elements, such as the pressure gauges, remain inaccessible for direct testing carried out at the time of use. Such a streamer section or case may be inserted when it is required but, if the number of sections or cases added is large, the total length of the streamer is increased very substantially and the modifications which result therefrom in the distribution of the sensors all along the streamer lead to a change in the seismic recordings obtained.
In another arrangement, measuring instruments such as depth sensors, are fixed to the outside of the flute, at evenly spaced intervals all along the length thereof and are associated respectively with inductive coupling transmission means for exchanging control and measurement signals with a transmission line inside the streamer, these means comprising transformers whose windings are disposed on each side of the outer sheath of the streamer. However, this presents a problem in that the induction connections require good magnetic coupling between the windings and, consequently, accurate positioning thereof with respect to each other is necessary.
The device of the invention overcomes the above mentioned disadvantages.